Average prices for 128GB solid-state drives (SSDs) hit something of a milestone in the second quarter of the year, falling to $50 for PC makers. And 256GB SSDs are now below the $100 mark.

According to DRAMeXchange data, the drop in SSD prices has been steady since early 2014. At that time, a 128GB SSD averaged $77.20 and the 256GB version sold for $148. (Note: Consumer prices for SSDs are dropping as well, but from a higher level, since OEMs buy in bulk and therefore pay less. The average retail price for a 128GB SSD is now just over $91.)

Jim Handy, principal analyst at Objective Analysis, attributed the lower prices to continued downward pressure on NAND flash memory, which accounts for 80% of the cost of an SSD drive. Flash prices have fallen by about 25% since June 2014, he said.

In fact, the data from DRAMeXchange shows that the price drop for internal SSDs is accelerating. Several factors are behind the ongoing decline:

Increased SSD adoption in laptops and other computer hardware, which spurs more production and economies of scale. SSDs are expected to account for 30% of the drives in laptops this year and 50% by 2017. On top of that, controller prices for the drives are falling, too, at a level that resembles Moore's Law, Handy said.

The move in recent years from flash that stores two bits per transistor to flash that stores three bits. As the density for NAND flash memory rises, SSDs at the same capacity cost less to make. Handy estimated that the conversion from two-bit, multi-level cell (MLC) flash to triple-level cell (TLC) flash has reduced costs about 20% over the past year.

The production of NAND flash has been migrating to 15- and 16-nanometer manufacturing processes (from 19-plus nanometer transistors in earlier generations). Again, that allows for more dense drives -- and lower production costs at the same capacity.

For computer users, the good news continues. In addition to cheaper SSD prices, manufacturers are moving to faster interfaces. The result: drives are not only likely to be cheaper in the years ahead, but faster, too.